Hoist rotary work can be cited as a typical work performed by a hydraulic shovel. In the hoist rotary work, earth and sand located below are loaded by a boom, an upper rotary body is rotated by a predetermined angle (for example, 90°) while the boom is being lifted, and the earth and sand are loaded on a loading platform of a dumper truck. During the hoist rotary work, the lifting of the boom and the rotation of the upper rotary body are simultaneously performed by a combined operation of a boom control lever and a rotary control lever.
A configuration of a conventional construction machine 1 will schematically be described with reference to FIG. 1. For the purpose of easy understanding, only the configuration used to actuate the upper rotary body and the boom is extracted and shown in FIG. 1. As shown in FIG. 1, a hydraulic pump 3 is driven using a diesel engine 2 which is a driving source. A variable displacement hydraulic pump is used as the hydraulic pump 3, and a volume q (cc/rev) is changed by changing an inclination angle of a swash plate 3a. Pressurized oil discharged from the hydraulic pump 3 at a discharge pressure Pp and a flow rate Q (cc/min) is supplied to a boom hydraulic cylinder 31 and a rotary hydraulic actuator 32 through a control valve 21 and a control valve 22 respectively. The control valve 21 and the control valve 22 are actuated by control levers 41 and 42. The hydraulic actuators 31 and 32 are driven by supplying the pressurized oil to the hydraulic actuators 31 and 32, thereby actuating the boom and upper rotary body which are connected to the hydraulic actuators 31 and 32.
Loads on the boom and upper rotary body are changed during the operation of the construction machine 1, which changes a load (hydraulic device load) on a hydraulic device (hydraulic pump 3), i.e., the load on the engine 2.
Load sensing control is performed to the hydraulic pump 3. That is, the inclination angle of the swash plate 3a is controlled such that pressure differences (pressure difference across control valve) ΔP between a discharge pressure Pp of the hydraulic pump 3 and load pressures (maximum load pressure) PLS of the hydraulic actuators 31 and 32 are kept constant.
Pressure compensation valves 51 and 52 are provided in the control valves 21 and 22 respectively such that a larger amount of pressurized oil is not supplied to the hydraulic actuator having a smaller load when the plural hydraulic actuators 31 and 32 are simultaneously actuated.
The pressure compensation valves 51 and 52 adjust the pressurized oil flowing into the control valves 21 and 22 such that the pressure differences ΔP across the control valves 21 and 22 are equal to each other. The pressure compensation valves 51 and 52 make the supply of the pressurized oil difficult by reducing the pressurized oil supplied to the control valve on the side on which the smaller load is applied.
A boom relief valve 61 is provided on a hydraulic passage connecting the control valve 21 and the hydraulic actuator 31. A rotary relief valve 62 is provided on a hydraulic passage connecting the control valve 22 and the hydraulic actuator 32. A setting relief pressure Prf of the rotary relief valve 62 is lower than a setting relief pressure of the boom relief valve. This is because the pressurized oil having a constant pressure is supplied to the hydraulic actuator 32 to improve operability during the rotation by performing relief actuation of the rotary relief valve 62 when the rotary control lever 42 is operated. For example, the relief pressure Prf of the rotary relief valve 62 is set to 270 kg/cm2 and the pressurized oil having the constant pressure of 270 kg/cm2 is supplied to the hydraulic actuator 32.
However, when the hoist rotary work is performed in the configuration, the following problems are generated.
1) Deterioration in Speed Matching Between Rotation and Boom
In rotating the hoist, ideally the speed matching is achieved between the upper rotary body and the boom when the control levers 41 and 42 are turned down to full lever positions, and the boom is lifted just to the height of the loading platform of the dumper truck when the upper rotary body is rotated to the loading platform of the dumper truck. Therefore, as shown in FIG. 2A, it is necessary that power (output, horse power; kW) of the engine 2 be appropriately distributed between the boom hydraulic actuator 31 and the rotary hydraulic actuator 32. When the power of the engine 2 reaches 100 kW, ideally the power of 30 kW in the total power of 100 kW is distributed to the rotary hydraulic actuator 32 and the residual power of 70 kW is distributed to the boom hydraulic actuator 31.
However, as described above, the pressurized oil of the relief pressure Prf (maximum pressure) is supplied to the rotary hydraulic actuator 32 during the hoist rotary work. This power distribution of the engine 2 is shown in FIG. 2B. That is, the power of 40 kW in the output of 100 kW of the engine 2 is distributed to the rotary hydraulic actuator 32 and the residual power of 60 kW is distributed to the boom hydraulic actuator 31.
In the power distribution of FIG. 2B, the power distribution on the upper rotary body side becomes excessively larger compared with the boom side, and the rotary speed of the upper rotary body becomes faster compared with the boom lifting speed. Therefore, sometimes the boom is not lifted to the height of the loading platform when the upper rotary body is rotated to the loading platform of the dumper truck. It is necessary for an operator to not perform the full lever operation of the control levers 41 and 42 but finely adjust the control levers 41 and 42 in order to achieve the speed matching between the boom and the upper rotary body. Accordingly, high skill is required for the operator and operability is decreased in rotating the hoist.
2) Deterioration in Energy Loss and Fuel Efficiency
As described above, during the hoist rotary work, the relief actuation is performed to the rotary relief valve 62 while the pressure compensation control of reducing the pressurized oil supplied to the control valve on the side of the lighter load is performed. Therefore, the excessive pressurized oil is discharged to a tank, which leads to energy loss or fuel inefficiency.
In order to solve such problems, there has been a technique which has been practically used and in which, during the hoist rotary work unlike the normal work, the pressure compensation control is stopped and the swash plate 3a of the hydraulic pump 3 is controlled based only on the load pressure of the boom hydraulic actuator 31 (conventional technique).
According to the conventional technique, the pressure corresponding to the load pressure of the boom hydraulic actuator 31, i.e., the pressurized oil (for example, 200 kg/cm2) lower than the relief pressure Prf (270 kg/cm2) is supplied to the rotary hydraulic actuator 32 during the hoist rotary work. Therefore, as shown in FIG. 2A, the power distribution of the engine 2 becomes close to the ideal distribution. When the control levers 41 and 42 are manipulated to the full lever position, the upper rotary body is rotated to the loading platform of the dumper truck, and the boom is lifted just to the height of the loading platform, which allows the substantially ideal hoist rotary work. The pressurized oil supplied to the control valve on the side of the lighter load is reduced due to the pressure compensation control, and the relief actuation of the rotary relief valve 62 is suppressed so that the problem of energy loss or fuel inefficiency is solved.
Patent Documents 1 and 2 also describe the technique in which the speed matching is achieved among the plural hydraulic actuators by adjusting the pressure of the pressurized oil supplied to each hydraulic actuator when the plural hydraulic actuators are actuated while combined with one another.
Patent Document 1: Japanese Patent Application Laid-Open No. 11-71788
Patent Document 2: Japanese Patent Application Laid-Open No. 2003-278705